Circasemidian rhythm

Numerous studies have demonstrated that human circadian rhythms in many measures of performance and physiological activity actually have a 2-peak daily (circasemidian) pattern.^[1][2][3] The name, circasemidian, is based upon the Latin words circa ("about"), semi ("half") and dia ("day"). Thus, this is a rhythm that has two cycles per day. It usually serves to (1) deepen the pre-dawn nadir in body temperature and cognitive performance, (2) create a flat spot during the early afternoon in the daytime increase in body temperature and cognitive performance (the "post-lunch dip"), and (3) heighten the early-evening peak in body temperature and cognitive performance. Broughton was the first to bring this characteristic of human performance to the attention of researchers.^[4][5][6]

No evidence exists to support the presence of a circasemidian rhythm in the rhythmic cells of the suprachiasmatic nucleus, the accepted internal timing source for the major circadian rhythms of the body. However, a number of published data sets have shown a daily two-peak error pattern in industrial and transportation environments.^{[7][8][9][10][11][12][13][14][15][16][17]} The pattern was also obvious in many of the charts shown in the review by Rutenfranz and Colquhoun,^[18] though they did not suggest a circasemidian rhythm as a mediator for the pattern. Other investigators have reported a circasemidian rhythm in body temperature,^{[19][20][21][22]} melatonin^[23] and slow-wave sleep.^[24]

These behavioral and physiological observations support the need to consider a 12-hour rhythmicity in the quantification of daily variations in physiological function and some kinds of cognitive performance in fatigue modeling efforts such as the Fatigue Avoidance Scheduling Tool.

References

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